There are three different situations / modes in a PVP multiplayer Rainbow Six: Siege session game: Bomb, Hostage, and Secure Area.
Defenders are to double-down in preventing the attacking team from extracting the hostage. The hostage is placed in same locations per map. Meaning, you only have to memorize three to five possible locations of the hostage and send your drone there in the preparing phase for confirmation. The attackers win when they extract or kill all defending teams. The defenders win if they finish off all of the attackers or the time expires. Whoever inadvertently or intentionally kills the hostage, will lose the round. To make it short, both sides should never kill the hostage no matter what.
A couple of bombs are placed in within an objective location. Same goes for the locations, they are pre-determined so you just have to memorize the possible locations. The defending team must prevent the attacking team from installing the defuser, and if the latter is successful the former should disarm the defuser. The defuser takes 60 seconds to disarm. Yes, feels like such a short-amount of time. But not really, that's a plenty of time. It takes three melee attacks to destroy the defuser. If the defuser disarms the bombs, the attacking team win. If not, obviously, the defending team wins. If the defuser is successfully deployed, even if the attacking team are all killed, the defending team must find the defuser and destroy it. Otherwise, killing the entire teams will make the surviving team the victor.
In some cases, it is actually possible to plant the defuser on windows, making it a very difficult for the defending team to destroy the defuser as they are going to be sniped out of existence from the outside. Also, try to hide the defuser in awkward locations so that the defending team will have a hard time finding it.
The modern version of King of the Hill. The defenders must find the biochemical agent and hold the position for at least 10 seconds. The timer does not start if both teams are within the same area. Both teams must drive the opposing team out of the area by death or by forcing them out. Destroying everyone of the opposing team will end the round with a victory for the surviving team.